Minimally Distinct Border (MDB)

A frequent method for psychophysically evaluating the amount of colored light is the method of heterochromatic flicker photometry (HFP). Flicker photometry introduces an artificial temporal component to the measurement. Most of the time when we compare the colors of multiple objects or areas the only temporal component is introduced by our eye movements. Subjectively the colors are continuously present and we make comparisons by shifting our attention from one area to another.

With the minimally distinct border (MDB) method the two fields to be compared are simultaneously present. As a result, the viewing situation is a little closer to average conditions.

As seen in the MDB demo figure the two fields being compared are placed in precise juxtaposition with each other. The crucial test is when both hemifields are identical a homogeneous circular field should be seen. This close juxtapositioning is not consistent with many natural viewing conditions but is crucial for the MDB method.

When half of the field is of a different color a border between the two hemifields is formed. As can be seen by noticing the change when going from Fig. A to Fig. B to Fig.C to Fig. D in the MDB demo the border becomes more distinct (stronger / salient) as the brightness of the color increases. A similar increase in border distinctness is observed as the variable field is made darker than the reference field.

In the laboratory this technique is implemented by presenting an observer with a bipartite field similar to the one seen in the MDB demo. The observer is asked to adjust the brightness of the variable field until the border between it and the reference field is minimally distinct. The reference field can be any spectral distribution. However, common practice is to make it achromatic.

To measure an observer's spectral sensitivity with the MDB method one uses a representative number of monochromatic wavelengths in the variable hemifield and determines the brightness of each required to obtain a minimally distinct border. When these brightnesses are known they are then measured radiometrically with the aid of a radiometer. The reciprocal of these energy measures are then plotted as a function of wavelength.

Measuring spectral sensitivity with the MDB method yields a function very close to that obtained by
heterochromatic flicker photometry. This correspondence suggests common visual mechanisms underlying MDB and HFP. This commonality has received support from various psychophysical and physiological experiments on man and monkey respectively. The interested reader can obtain more information on this subject in Human Color Vision,